Since you are just using authorize.net, why not use a library specifically designed for the Authorize.net API and forget about constructing your own XML calls?
If you want or need to go your own way with XML, don't use minidom, use something with an ElementTree
interface such as cElementTree
(which is in the standard library). It will be much less painful and probably much faster. You will certainly need an XML library to parse the XML you produce, so you might as well use the same API for both.
It is very unlikely that the overhead of using an XML library will be a problem, and the benefit in clean code and knowing you can't generate invalid XML is very great.
If you absolutely, positively need to be as fast as possible, use one of the extremely fast templating libraries available for Python. They will probably be much faster than any naive string concatenation you do and will also be safe (i.e do proper escaping).
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